My long-term career goal is to establish an independent research program at a strong academic institution with emphasis on important basic and biomedical AIDS research problems. I am particularly interested in novel AIDS vaccine approaches, since I believe a safe and effective vaccine represents the greatest hope for controlling the spread of HIV. The research activities described in this RSDA proposal are intended to provide the framework for transition to an independent faculty position. Current AIDS vaccine approaches based on live-attenuated viruses or non-persisting recombinant vectors have drawbacks in terms of safety and efficacy that will likely prevent their widespread use in humans for the foreseeable future. Thus, the search continues for an approach with a more optimal safety and efficacy profile. One strategy that has not yet been explored is the use of lentiviruses that are limited to a single cycle of infection. A lentivirus that is limited to a single cycle of integration and protein expression should stimulate a broad spectrum of virus-specific antibody and cellular immune responses, but should not be capable of causing AIDS in immunized individuals. To test this hypothesis, we have developed a novel system for producing single-cycle SIV that minimizes the chances of generating replication-competent virus through recombination or nucleotide reversion. In the first specific aim, we will compare the strength and stability of virus-specific immune responses elicited by intravenous inoculation of macaques with T cell-tropic versus macrophage-tropic single-cycle SIV. In the second specific aim, we will determine whether the selective elimination of N-Iinked glycosylation sites in gp120 can enhance neutralizing antibody responses elicited by single-cycle SIV. In the third specific aim, we will determine whether repeated vaginal exposure to single-cycle SIV can induce mucosal antibody and cellular immune responses. Macaques immunized with single-cycle viruses will be challenged with wild type SIV by mucosal routes to assess protection.